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TPS25750: question on current available from LDO_3V3

mbbrouwer
Expert 3650 points
Part Number: TPS25750

HI there,

Working through some questions

From Figure 9-11 in the datasheet, during “dead battery” mode, VIN_3V3 is not available & LDO_3V3 is driven by the LDO from VBUS.

From spec table in section 7.3.1able below, the output current from VBUS_LDO is 5mA max (current from LDO_3V3)

On the eval board shown below, the LDO_3V3 supply is used for:

  • The pull ups on the I2Cslave & I2Cmaster signals
  • To feed the EEPROM

EEPROM on the EVM CAT24C256 has the following current consumption

Read = 1mA

Write = 3mA

Writing to the EEPROM via I2Cm

  1. If both SCL & SDA are low, the 2.2K to LDO_3V3 will consume 2x(3.3/2200) = 3.0mA
  2. If there is an interrupt, IRQ this will consume, 3.3/10000 = 0.33mA
  3. The EEPROM will consume 3mA max

The total of all these is already at 6.33mA which exceeds the max of 5mA shown on the TPS25750 3V3 LDO Spec

Reading from the EEPROM via I2Cm

  1. Same as above
  2. Same as above
  3. The EEPROM will consume 1 mA

The total of all these is already at 4.33mA which is quite close to the max of 5mA

MAIN QUESTION: Is the 5mA for 3V3 LDO correct and if it is, the Pull Ups for the I2Cm should be increased in order to reduce the loading.

SECOND QUESTION:

In Spec Section, Spec Table shows “1mA max for GPIOx” does this mean:

  • 1mA for each GPIO individually ?
  • A total of 1mA max available for all 11 GPIOs together ?

Thanks!

  • 0
    TI__Mastermind 32711 points

    Marc,

    You calculations are correct if you pick the absolute worst case for every computation.  Remember that pull-ups only consume current when they are being driven low and you can reduce the I2C pull-up resistors as long as the bus capacitance is low.  I do not recommend going any larger than 4.7K because even with low capacitance on the I2C bus, you will start to see issues at 400kHz switching.

    For Calculation sake

    I would assume that SCL and SDA are high half the time which cuts the current draw to 1.5mA.  This is actually a very pessimistic number.  It will likely be closer to 900uA because of the I2C protocol and the odds of ones in the data stream, but let's use 1.5mA for the computation.

    The IRQ line will not be active at all because it is disabled during boot

    The EEPROM number is 3mA max.

    In this case, for  I2C boot, the total current draw will be 4.5mA worst case which fill fit in the budget and as I said, I believe that this will most likely be closer to 4mA because of I2C protocol reasons.

    The next question is how the GPIO current will impact the LDO_3V3.

    The GPIO will not draw DC current unless it is driving against a DC load.  This will be the cause when there is a pull up of if you attempt to Drive a LED directly which we strongly recommend against. Because of our lack of current capability in our LDO.

    The GPIO's are specified to be able to meet their VOL specification under 1mA of load.  This is not a recommendation of how much load to put them under.

    The GPIO on the TPS25750 are intended to work with very light pull-ups and not to drive LED directly.  If these conditions are met, then the LDO_3V3 is sufficient to provide power